It has been customary in the electronic article surveillance (EAS) industry to apply to articles to be monitored either hard, reusable EAS tags or disposable adhesive EAS labels, both functioning as article monitoring devices. A checkout clerk passes the article over or into deactivation apparatus which deactivates the monitoring device.
Known deactivation apparatus includes coil structure energizable to generate a magnetic field of magnitude sufficient to render the monitoring device inactive, i.e., no longer responsive to incident energy itself to provide output alarm or to transmit an alarm condition to an alarm unit external to the tag or label (hereinafter "tag").
One commercial deactivator of the assignee hereof employs one coil disposed horizontally within a housing and tagged articles are moved across the horizontal top surface of the housing such that the tag is disposed generally coplanarly with the coil.
Another commercial deactivator of the assignee hereof employs a housing having an open side with a plastic bucket inserted in the housing such that an article or a plurality of articles may be made resident in the bucket. Three coil pairs are disposed about the bucket in respective x-, y- and z-axis planes, whereby orientation of the tag as in the first discussed deactivator is not required.
The prior art has come to appreciate the desirability of the amplitude envelope of the current used for driving deactivating coils shaped so as to decay linearly from a given maximum to zero in a specified time interval resulting in a given decay rate. The magnetic field generated will exhibit the same shape as the driving current, i.e., a sawtooth current waveform, the peaks of which define the desired amplitude envelope.
One known past approach to driving current amplitude envelope shaping involves an elaborate microprocessor based system. An H-bridge of power transistors is employed to drive the deactivating coil at levels of tens of amperes. Current values are measured and compared with a digital look up table of peak current values defining the desired current amplitude envelope. The H-bridge transistors are controlled in accordance with the comparison results by digital switching.
From applicant's perspective, the prior art approach is unduly complicated and costly. Further, significant and undesired electronic noise is generated therein due to the microprocessor and associated digital switching.